The present invention relates to a manufacturing method of a compound semiconductor wafer made of a compound semiconductor including nitrogen in its composition and used in a short wavelength laser, a high-temperature operating transistor, etc.
Conventionally, it has been known that a compound semiconductor including at least one element selected from Ga, Al, B, As, In, P and Sb and N (hereinafter, referred to as nitride semiconductor) has broad bandgap energy from the ultraviolet region to the visible region, and therefore, is a potential semiconductor material for light emitting and light receiving devices. A typical example of the nitride semiconductor is a compound semiconductor expressed by a general formula BxAlyGazIn1-x-y-zN, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and 0xe2x89xa6x+y+zxe2x89xa61. Recently, there has been an increasing demand for a good-quality and large-area nitride semiconductor wafer, and most of all, a free-standing wafer (self-standing wafer), realized as an underlying substrate used when manufacturing a device from the nitride semiconductor.
The free-standing nitride semiconductor wafer is a wafer made of a nitride semiconductor alone excluding any material other than the nitride semiconductor. Generally, in order to obtain the free-standing nitride semiconductor wafer, a method of conducting epitaxial growth of a nitride semiconductor film on a substrate made of a material other than the nitride semiconductor and removing the substrate later is used. As is disclosed in, for example, U.S. Pat. No. 6,071,795, a technique of irradiating a beam of laser light from an excimer KrF laser or a Nd/YAG laser to the substrate from the back surface thereof (laser lift-off) has been known as one of the methods of removing the substrate.
FIGS. 13A and 13B are cross sections showing the steps of forming a conventional free-standing nitride semiconductor film.
Initially, a sapphire substrate 101 (for example, a sapphire wafer measuring two inches across), which is transparent with respect to laser light from the excimer KrF laser or Nd/YAG laser, is prepared. Then, the sapphire substrate 101 is placed in a hydride vapor phase epitaxy (hereinafter, abbreviated to HVPE) apparatus.
Subsequently, in the step shown in FIG. 13A, a nitride semiconductor film 102, for example, made of GaN and having a thickness of approximately 300 xcexcm, is formed on the sapphire substrate 101 through HVPE. At this point, the nitride semiconductor film 102 has a plane portion 102a located on the top surface of the sapphire substrate 101 and a side surface portion 102b located on the side surface of the sapphire substrate 101.
Then, strong laser light having, for example, a wavelength of 355 nm, is irradiated to the sapphire substrate 101 from the back surface thereof. Because the sapphire substrate 101 transmits light and the irradiated laser light has an extremely short pulse width, the laser light is absorbed in the plane portion 102a of the nitride semiconductor film 102 only at a region in contact with the sapphire substrate 101, that is, the back surface portion. As a result, the back surface portion of the plane portion 102a of the nitride semiconductor film 102 is heated, and undergoes heat dissociation to be decomposed to gallium and nitrogen, whereupon a nitrogen gas is released. Thus, the nitride semiconductor film 102 separates from the sapphire substrate 101 as laser light scans the surface of the sapphire substrate 101 entirely. Then, by removing the sapphire substrate 101, the nitride semiconductor film 102 that will be made into a free-standing nitride semiconductor wafer can be obtained. Thereafter, one or two or more crystalline layers of a compound semiconductor including at least one element selected from Ga, Al, B, As, In, P and Sb and N in its composition (a compound semiconductor expressed by a general formula BxAlyGazIn1-x-y-zN, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and 0xe2x89xa6x+y+zxe2x89xa61) are grown through epitaxial growth on the nitride semiconductor film 102, whereby various kinds of compound semiconductor devices can be obtained.
As another method of obtaining the free-standing nitride semiconductor wafer, there also has been known a method of obtaining a nitride semiconductor film that will be made into the free-standing nitride semiconductor wafer by mechanically polishing the sapphire substrate 101.
As a still another method of obtaining the freestanding nitride semiconductor wafer, there has been attempted a method of using a GaAs substrate, an Si substrate, etc., which are materials that can be readily removed by means of etching, instead of the sapphire substrate, so that the GaAs substrate or Si substrate is removed not by means of polishing but by means of wet etching after the epitaxial growth of the nitride semiconductor film through HVPE.
However, the above conventional methods have inconveniences as follows.
That is, according to the method shown in FIGS. 13A and 13B, of the entire nitride semiconductor film 102, the back surface portion of the plane portion 102a is decomposed by irradiation of laser light, so that it is relatively easy to separate the plane portion 102a of the nitride semiconductor film 102 from the sapphire substrate 101. However, because laser light is hardly irradiated to the side surface portion 102b of the nitride semiconductor film 102, it is generally difficult to decompose a region in the vicinity of the interface of the side surface portion 102b and the sapphire substrate 101. Hence, for example, if the sapphire substrate 101 and the nitride semiconductor film 102 are heated in trying to separate the nitride semiconductor film 102 and the sapphire substrate 101 from each other, a difference in coefficients of thermal expansion between the side surface portion 102b of the nitride semiconductor film 102 and the sapphire substrate 101 produces stress, which is applied intensively to the side surface portion 102b of the nitride semiconductor film 102. Accordingly, a crack occurs on the side surface portion 102b, which may possibly result in a braking of the plane portion 102a of the nitride semiconductor film 102.
And, the crystal orientation differs in the sapphire substrate 101 between the side surface portion and the top surface portion. Moreover, crystallinity is disturbed by machining treatment or the like during the manufacturing of the substrate, and the crystallinity at the side surface portion 102b of the nitride semiconductor 102 is so poor that there is a portion having an almost polycrystalline structure. For this reason, the side surface portion 102b of the nitride semiconductor film 102, in general, readily causes a breaking or a chipping, which becomes one of the factors of the occurrence of inconveniences.
Also, according to the method of removing the sapphire substrate 101 by means of polishing, the side surface portion 102b of the nitride semiconductor 102 is also polished while the sapphire substrate 101 is polished. Hence, mechanical stress readily causes a crack or a breaking that would run up to the plane portion 102a of the nitride semiconductor film 102 from a point where such a crack or breaking caused first on the side surface portion 102b. Thus, if this method is used, it is difficult to obtain an independent large-area nitride semiconductor film 102 with satisfactory reproducibility.
Also, according to the method of using the GaAs substrate or Si substrate and removing the GaAs substrate or Si substrate not by means of polishing but by means of etching, the nitride semiconductor film 102 readily breaks when handled in the form of a wafer after the substrate is removed, and a significant crack or breaking readily occurs in the nitride semiconductor film 102 from a point where it breaks first. Hence, if this method is used, it is also difficult to obtain an independent large-area nitride semiconductor film 102 with satisfactory reproducibility.
Further, an independent nitride semiconductor wafer (nitride semiconductor film 102) obtained in the above manner is generally subjected to surface polishing treatment before a semiconductor device or the like is formed thereon. However, the side surface portion 102b readily breaks also by mechanical stress during the surface polishing step, and the entire wafer may possibly break from a point where the breaking caused first.
Even in a case where the entire wafer does not break, a crack may remain within the wafer by excessive mechanical stress applied during the separating or polishing step. In case that a semiconductor element, such as a field effect transistor, an LED, and a laser diode, is formed on the nitride semiconductor wafer having a remaining crack therein, the remaining crack may cause leakage of a current and reduces the reliability, or the crack may scatter light around it and reduces a light emitting efficiency.
It is therefore an object of the present invention to provide a manufacturing method of a compound semiconductor wafer, by which a free-standing large-area nitride semiconductor wafer can be obtained at a high yield and with satisfactory reproducibility.
A first manufacturing method of a compound semiconductor wafer of the present invention includes: a step (a) of forming a closed-ring protector film covering a part of a top surface and a side surface of a substrate; a step (b) of conducting, after the step (a), epitaxial growth of a compound semiconductor film including nitrogen in a composition thereof on the top surface and the side surface of the substrate at a region where it is not covered with the protector film; and a step (c) of removing the substrate after the step (b), wherein the protector film formed in the step (a) has a function of interfering with the epitaxial growth of the compound semiconductor film formed in the step (b).
According to this method, it is possible to use, of the entire compound semiconductor film, only a portion grown through epitaxial growth from the top surface of the substrate as a free-standing wafer. Also, because this portion has no portion grown from the side surface of the substrate, it is possible to control the occurrence of a crack or a chipping in the latter steps. Further, it is possible to obtain a good-quality wafer that is hardly influenced by changes of the epitaxial growth conditions in the vicinity of the side surface of the substrate.
In the step (a), by forming the protector film so as to at least cover the side surface of the substrate entirely, it is possible to form the compound semiconductor film from only a portion grown through epitaxial growth from the top surface of the substrate, and as a result, the foregoing advantages can be exerted in a reliable manner.
In the step (a), by forming the protector film so as to cover only a part of the top surface of the substrate, a compound semiconductor film grown through epitaxial growth from the top surface of the substrate and a compound semiconductor film grown through epitaxial growth from the side surface of the substrate are obtained, and only the former can be used as a free-standing wafer.
In the step (a), by forming the protector film so that a minimum value of a ring width of the protector film is larger than a film thickness of the compound semiconductor film, it is possible to separate a compound semiconductor film grown through epitaxial growth from the top surface of the substrate from a compound semiconductor film grown through epitaxial growth from the side surface of the substrate in a reliable manner.
It is preferable that the protector film is made of at least one film selected from a silicon dioxide film, a silicon nitride film, a silicon oxynitride film, and a refractory metal film.
In the step (c), the substrate may be removed by means of polishing.
In the step (b), the compound semiconductor film is formed from a compound semiconductor having an absorption edge wavelength longer than an absorption edge wavelength of the substrate, and in the step (c), by irradiating light having an intermediate wavelength between the absorption edge wavelength of the substrate and the absorption edge wavelength of the compound semiconductor film from the substrate side, it is possible to decompose a part of the compound semiconductor film and thereby to separate the substrate from the compound semiconductor film.
In the step (c), the substrate may be removed by means of etching.
Also, it is preferable to polish a back surface of the compound semiconductor film after the step (c).
A second manufacturing method of a compound semiconductor wafer of the present invention includes: a step (a) of conducting epitaxial growth of a compound semiconductor film including nitrogen in a composition thereof on a substrate; a step (b) of removing at least a portion of the compound semiconductor film located on a side surface of the substrate; and a step (c) of removing the substrate after the step (b).
According to this method, it is possible to use, of the entire compound semiconductor film, only a portion grown through epitaxial growth from the top surface of the substrate as a free-standing wafer. Also, because this portion has no portion grown from the side surface of the substrate, it is possible to control the occurrence of a crack or a chipping in the latter steps. In particular, a portion influenced by changes of the epitaxial growth conditions in the vicinity of the side surface of the substrate can be removed in a more reliable manner, a good-quality wafer can be obtained.
In the step (b), at least the portion of the compound semiconductor film located on the side surface of the substrate may be removed by means of polishing.
In the step (b), a portion of the substrate and the compound semiconductor film located a certain distance inside the side surface is cut in a closed-ring shape.
In the step (b), an inside portion of the compound semiconductor film up to a certain distance from the side surface may be removed.
In the step (b), it is preferable to form a compound semiconductor film including at least one element selected from Ga, Al, B, As, In, P and Sb, and N in its composition as the compound semiconductor film.
The manufacturing method of the second compound semiconductor wafer can also adopt the above-discussed preferred embodiments adopted by the manufacturing method of the first compound semiconductor wafer.
A third manufacturing method of a compound semiconductor wafer of the present invention includes: a step (a) of depositing a film covering a top surface and a side surface of a substrate; a step (b) of removing the film until at least the top surface of the substrate is exposed, thereby flattening the substrate and the film at the top to form a closed-ring protector film covering at least the side surface of the substrate; a step (c) of conducting, after the step (b), epitaxial growth of a compound semiconductor film including nitrogen in a composition thereof on the top surface of the substrate at a region where it is not covered by the protector film; and a step (d) of removing the substrate after the step (c). The protector film formed in the step (b) has a function of interfering with the epitaxial growth of the compound conductor film formed in the step (c).
According to this method, only a portion of the compound semiconductor film that is epitaxially grown from the top surface of the substrate can be used as a freestanding wafer. Since this portion has no part grown from the side surface of the substrate, it is possible to control the occurrence of a crack or a chipping in the latter steps. In addition, it is possible to obtain a wafer of good quality that is less susceptible to variations in the epitaxial growth condition around the side surface of the substrate.
The step (a) may be preceded by an additional step of removing a peripheral portion of the substrate to a certain depth to form a notch. In the step (b), the protector film is then formed so as to cover the side surface and the notch of the substrate. In such a case, it is possible to obtain a wafer of good quality that is still less susceptible to variations in the epitaxial growth conditions around the side surface of the substrate, aside from the foregoing effect.